Acoustical enclosures



L. BARTHEL May 16, 1967 ACOUSTICAL ENCLOSURES Filed June 17, 1966 /m e/1lor [ac/en B rf/rel 5 (5. T0 1500a 9w Attorney United States Patent 3,319,737 ACUUSTICAL ENCLOSURES Lucien Barthel, 54 Rue de Rome, Paris, France Filed June 17, 1966, Ser. No. 558,403 8 Claims. ((31. 181-31) The present invention relates generally to an improved type of acoustic enclosures.

It is known, for the reproduction of sounds, to make use of a loud-speaker or, more generally, of an electroacoustic transducer mounted in the opening of an acoustical chamber or encolusure.

A great many types of such acoustic enclosures are presently known which employ means effective mainly to avoid any natural resonance, i.e. to reinforce, in the frequency spectrum, certain preferential frequencies or to suppress certain others.

To this end, sound-absorbing walls or walls lined with sound-absorbing substances, such as felt, are frequently made use of, imparting to said enclosures a complex shape adapted, in particular, to avoid standing waves. Moreover, certain partitions of these enclosures are more or less filled up with sound-absorbing substances, such as glass wool. All these arrangements, which present the disadvantage of absorbing a large proportion of the acoustic energy, do not give, in use, entirely satisfactory results.

It is an object of the invention to provide an improved acoustic enclosure which does not bring about virtual dissipation of the sound energy.

It is a further object of the invention to provide an enclosure exhibiting a response curve as uniform and as horizontal as possible, i.e. a sound level substantially equal for all the frequencies of the sound spectrum.

It is another object of the invention to improve the existing acoustic enclosures by adjoining, inside thereof, an element which does not modify the aspect of the enclosures.

Still another object of the invention is to achieve the broadening of the response spectrum of a sound-reproducing system.

According to the invention, in the acoustic enclosure of a sound source there is secured a closed box having a thin wall of rigid material formed with at least one opening, said box having rigidly fixed to a wall surface thereof one end of an oblong plate or reed of rigid and light material adapted to vibrate transversely, on its free end, with respect to its fixed extremity.

The box with its vibrating reed constitutes a kind of aperiodic resonator, i.e. one without a natural frequency, which converts into solid vibrations and vibrations of the air in the enclosure and, thereby, will render uniform the intensity of these vibrations.

Preferably, the reed, of a somewhat smaller length than the box, is placed inside this box.

The invention will be best understood from the following description and the appended drawing wherein:

FIG. 1 is a front view of a sound-reproducing device according to the invention;

FIG. 2 is a cross-sectional view taken on line II-II of FIG. 1;,

FIG. 3 is an alternative embodiment of the resonator to be incorporated into an acoustical enclosure;

FIG. 4 is a perspective View of a further alternative embodiment of such a resonator;

FIG. 5 is a vertical cross-sectional view of another type of sound reproducer;

FIG. 6 is a front view of the inner resonator of the acoustic enclosure illustrated in FIG. 5; and

FIG. 7 shows two response curves of the same sound reproducer, the one in dotted lines being taken in the absence of an inner resonator, the other being taken with such a resonator.

The sound reproducer shown in FIGS. 1 and 2 comprises an acoustic enclosure 1 which is shaped as a closed parallelpipedic box, of a relatively thick and non rigid material, for example a laminated or plastic material. In an opening of this enclosure there is mounted an electro-acoustic transducer, for instance an electrodynamic loudspeaker 2, with a conical diaphragm 3.

Inside the enclosure there is secured a resonator 4. This resonator is an oblong box with a thin, rigid and resonant wall, of a sound plastic material such as methylmethacrylate. As shown, the walls of the prismatic box 4 are parallel to those of the prismatic resonator chamber 1.

By means of a screw 5, a flat rod or plate 6 is secured to Wall 411 of box 4 so as to bear against a crossbeam 8 (somewhat similar to the nut of a violin) while its end portion 6a rests against wall 7. The free'extremity 6b of the reed or strip 6 may thus vibrate in box 4 and, through said cross-beam 8, may transmit its vibration to box 4 and vice versa. The tightening of screw 5 increases the stresses of the plate or reed and, thus, its vibrating capacity.

Plate or strip 6 is, preferably, cut from a board of light, fibrous wood, such as pine wood, and is, preferably, varnished so that the propagation velocity of the sound in said plate should be as high as possible, which, as well known in the stringed-instrument industry, favors its vibration capacity. Specifically, the plate or reed may have a thickness in the range between 6 and 12 mm. (M1 to /2 inch) and a width of 2.5 to 5 cm. (1 to 2 inches); its length may be 10 to 50 cm. (4 to 20 inches). The thickness of box 4 is, preferably, lower than 5 mm. inch).

The dimensions of box 4 fit the location of the plate, i.e. the length of the box is slightly greater than that of the plate, its width is 3 to 4 times that of the plate and its depth enables the plate or reed to vibrate without striking the walls.

By means of screws 10, box 4 is secured to wall 1a of the enclosure 1. Finally, one at least of the two walls of box 4 opposite the faces of plate 6 is formed with at least one opening 11 whose shape may be as desired. When this opening is large, it may be closed by means of a diaphragm 12 formed of a flexible, plastic, cellular material, thus limiting the energy transmission through opening 11.

When a sound-reproducing apparatus is used without any resonator inside, the response curve of the apparatus will be, for instance, that shown at 13 in FIG. 7. As is well known, this response curve is generally ascending, from the low sounds up to the high-pitched sounds, and comprises peaks 13a and pits or valleys 13b corresponding to the sound frequencies which are, respectively, intensified or suppressed.

When the resonator is secured inside the apparatus, the sound-response curve is that indicated at 14. This curve is substantially horizontal and presents but weak excursions, moreover, the low frequencies (lower than 50 Hz.) which were not emitted by the apparatus without resonator are now reproduced with a normal sound intensity.

This state of uniformity presented by the response curves results, for instance in rendering perfectly comprehensible (as in direct reception) the singing voice, while causing objectionable resonances, such as those known as barrel effect or as the diaphragm noise, or other random noises to be suppressed.

No tentative complete explanation of the experimental result obtained will be provided herein. It will be, however, noted that the resonator with its inner plate or reed behaves like a sporadic resonator or sound-box, similar to a violin case, and that, located inside a substantially closed acoustic chamber, it is caused to vibrate by the whole amount .of sound energy diffused in that chamber. It thus constitutes a solid relay in the transmission of sound energy outwards and, since its transmission capacity is constant, there results a uniformized state of the loud-speaker and its radiated-energy load.

In addition, since the low frequencies are less absorbed than the higher frequencies, there is obtained a correction of the natural tendency of such apparatus to promote the higher frequencies.

Compared to known sound-reproducing apparatus, wherein a correct response curve is obtained only by absorbing part of the sound energy, this result is obtained, according to the invention, by allowing all sound frequencies to develop at the same energy level. Consequently, it is possible to overload the transducer without interfering with the distribution of the sounds, which cannot be achieved with conventional apparatus.

Finally, a single electro-acoustic transducer may suffice to provide a satisfactory reproduction of sound recordings; it is obviously also possible to arrange for a plurality of electro-acoustic transducers to be mounted in the same enclosure. Moreover, the enclosure may also be constituted by the case of a stringed musical instrument: it is for instance also feasable to manufacture double-basses having a smaller size than the stringed double-basses presently in use.

In the embodiment illustrated in FIG. 3, the plate or reed 6 is secured through a screw and a cross-beam 8 to the outer surface of wall 4a of a box 4; the transmission of the vibrations of the plate to the box may then be effected by a connecting rod 14 (here shown as a screw) which may be mounted under tension or compression between plate 6 and wall 4a, so as to allow the stress condition of the plate to be varied. Finally, to transmit the vibrations from wall 4a to wall 4b, a rod 15, also held under tensile or compressive stress between these walls, may be added. This rod may be of wood or similar material and can be likened to the part of a violin called a soundpost.

Such a rod 15 may also be provided in the other embodiments of a resonator already described and illustrated, wherein the plate or reed 6 is arranged inside box 4.

In the embodiment of FIG. 4, box 4 is of a molded plastic material; plate 6 is secured to a rigid base-plate 16 carrying the nut 8, this base-plate serving to transmit the vibrations and being attached to an inner wall of box 4, for instance by bonding. A bottom 17 closes the opening of the box and the wings 18 .enable its'fixation inside the sound enclosure.

In the embodiments shown in FIGS. 5 and 6, the two opposite walls 4a and 4b of box 4 are each provided with two openings in the shape of longitudinal slots 19, said slots defining, in each one of these faces, an intermediate longitudinal band 20 integral with the respective wall. Plate or reed 6 is secured to each one of said bands 20 through connecting rods 21 and 22, said plate being still arranged obliquely in box 4. These rods are threaded so as to be tensioned by screwing, in such a manner that, as illustrated, the bands 20 are stretched and curved inwardly and the plate or reed 6 is strongly submitted to a flexural stress. As shown in the drawing, the rods 21, 22

linking the reed 6 with the bands 20 are relatively offset along the reed. In this case, the box 4 is secured by means of lugs 23, extending longitudinally beyond its transverse end portions, to a wall of the sound enclosure.

In order to improve the performance of an existing acoustic chamber, it is possible to mount therein a resonator device such as those illustrated in FIGS. 3, 4 and 6.

While in any kind of enclosure certain locations are to be preferred, a substantial improvement may be obtained whatever the location selected. It is thus possible to transform a poor type of sound reproducer, by incorporatingtherein a practically closed acoustic enclosure according to the invention, into a satisfactory apparatus.

It will be noted that in FIGS. 1, 2 and 5 the positions of sound generators 2, 3, reed 6 and wall opening 11 or 19 are so chosen that the reed is shielded against direct transmission of vibrations from the loud-speaker.

What I claim is:

1. An acoustic device comprising a substantially closed resonator chamber with sound-generating means therein, a rigid, thin-walled box in said chamber spaced from said sound-generating means and provided with at least one wall aperture which opens into said chamber for the transmission of acoustic energy between the air within said box and the surrounding atmosphere inside said chamber, said box being substantially closed otherwise, and a vibra tile reed in said chamber mounted on a wall surface of said box in vibration-transmitting relationship with a wall thereof.

2. A device as defined in claim 1 wherein said reed is disposed inside said box at a location shielded against direct trans-mission of vibrations from said sound-generating means.

3. A device as defined in claim 1 wherein said chamber and said box are both of prismatic configuration with mutually parallel walls.

4. A device as defined in claim 3 wherein said reed extends obliquely within said box between a pair of parallel longitudinal walls thereof with a free end spaced from both said walls and with stressed link means connecting an intermediate portion of said reed with at least one of said walls.

5. A device as defined in claim 4 wherein said aperture is provided in at least one of said longitudinal walls.

6. A device as defined in claim 5 wherein said aperture is obstructed by flexible diaphragm means.

7. A device as defined in claim 5 wherein said aperture is defined by a pair of parallel slots separated by a band integral with the corresponding wall, said band being tied to said reed by said link means.

8. A device as defined in claim 7 wherein each of said longitudinal walls has an aperture spanned by a band integral with the respective wall, said link means comprising a pair of relatively offset tension members respectively connecting said reed with said bands.

References Cited by the Examiner UNITED STATES PATENTS 2,811,215 10/1957 Rudd et a1 181-31 2,839,150 6/1958 Perlman 181-31 3,109,509 11/1963 Klug 181-31 3,143,182 8/1964 Sears et a1. 181-31 STEPHEN I. TOMSKY, Primary Examiner. 

1. AN ACOUSTIC DEVICE COMPRISING A SUBSTANTIALLY CLOSED RESONATOR CHAMBER WITH SOUND-GENERATING MEANS THEREIN, A RIGID, THIN-WALLED BOX IN SAID CHAMBER SPACED FROM SAID SOUND-GENERATING MEANS AND PROVIDED WITH AT LEAST ONE WALL APERTURE WHICH OPENS INTO SAID CHAMBER FOR THE TRANSMISSION OF ACOUSTIC ENERGY BETWEEN THE AIR WITHIN SAID BOX AND THE SURROUNDING ATMOSPHERE INSIDE SAID CHAMBER, SAID BOX BEING SUBSTANTIALLY CLOSED OTHERWISE, AND A VIBRATILE REED IN SAID CHAMBER MOUNTED ON A WALL SURFACE OF SAID BOX IN VIBRATION-TRANSMITTING RELATIONSHIP WITH A WALL THEREOF. 